From my experience, and that of various friends who have sold an instrument through them, they tend to sell it for more than you might on here, helping to mitigate the commission fee. They’re also responsible for the shipping, and completing customs paperwork, and for any after sale service.
They’re a long-established shop with a good international reputation. It makes sense - to me at least - if you’ve got something a little out of the ordinary, to use them. They have a wider customer base to sell to. If you look at the BC marketplace, there’s some instruments that have been for sale for absolutely ages.